1. Field of the Invention
The present invention relates to a field effect transistor-type sensor for detecting a variation of the gate operation of a field effect transistor due to a variation of external factors, by a sensitive means formed on the gate insulating film of an MOS- or MIS-field effect transistor device.
2. Description of the Prior Art
A field effect transistor (hereinafter, referred to as FET)-type sensor, which comprises an FET device incorporated with a sensitive means exhibiting an electric variation of electrostatic capacity, electric conductivity, electrostatic potential, etc., due to a physical or chemical interaction with the physical quantity to be detected, detects the said physical quantity as a variation of the gate operation of the said FET device. Taking advantage of the high input impedance and the amplifying function of the FET device, such an FET type sensor can exhibit a high output, even though its size is extremely small, and thus is advantageous in actual use. Particularly an FET type sensor which is constructed in such a manner as to have a sensitive means on the gate isulating film of the FET device is advantageous practically and economically since the FET device can be small, and a number of devices can be formed on the same substrate. However, such an FET type sensor containing the FET device therein is inferior to an ordinary FET device alone in the operation stability of the FET device. It is also inferior to an FET device in the output stability and the reproducibility of the output characteristic. Depending upon the kind of the FET type sensor required, materials and production processes of the sensitive means are so different that the operation characteristic of the FET device can be remarkably varied. As compared with an ordinary FET device, a large amount of impurities and/or ions are apt to appear in the sensitive means or may contaminate the interface between the sensitive means and the gate insulating film during the formation of the sensitive means on the FET device, causing instability not only in the operation characteristic of the FET device but also in the output characteristic of the FET type sensor. Moreover, since the FET type sensor, which is designed to be used as an atmosphere sensor such as a gas sensor, a moisture sensor, etc., is exposed to an atmosphere, it will be contaminated by impurities in the atmosphere, causing variation and/or deterioration of the FET characteristic and/or deterioration of the sensor itself. Accordingly, an FET type sensor must be provided with an arrangement which will suppress the influence of impurities and/or ions contained in the materials of the sensitive means, or impurities and/or ions which contaminated the interface between the sensitive means and the gate insulating film during the formation of the sensitive means on the FET device and/or during operation of the FET device, thereby providing for a stable output characteristic over a long period of time. If such an FET type sensor is designed, a variety of sensors such as gas sensors, moisture sensors, ion sensors, biological sensors, infrared-ray sensors, etc., will be able to be produced in an FET type format. FET type gas sensors, moisture sensors, ion sensors and biological sensors cannot avoid direct interaction of the sensitive means with the atmosphere so that the device therein cannot be covered with a package, etc. Therefore, the above-mentioned problems deriving from the contamination, etc., of impurities and/or ions from the outside must be solved for FET type sensors. To solve these problems, a silicon nitride film having a small diffusion coefficient for ions, moisture, etc., has been used as the gate insulating film, or used to cover the surface of the FET device. The resulting FET sensors are, however, still inferior in output stability over a long period of time.
In order to solve the above-mentioned problems, the present applicant has proposed an FET type sensor having a double gate-electrode structure which was disclosed in U.S. patent application Ser. No. 697,640 and British Patent Application No. 8503061. This FET type sensor is excellent in output stability over a long period of time, but the following two problems still remain: One of them is that the characteristics of the FET device vary widely resulting in a wide variation of the sensor output. To correct this phenomenon, the selection of devices and/or the regulation of circuits is essential. The other is that the characteristics of the FET device depend upon temperature resulting in a variation of the output, and specific circuits are required to correct this phenomenon.
The following explanation of the operation of the above-mentioned FET type sensor proposed by the applicant makes possible a comprehensive understanding of these problems of the said FET type sensor:
As shown in FIG. 5, the FET type moisture sensor comprises an FET device 11 incorporated with a moisture sensitive means 9.
The FET device 11 is an MOS-type n-channel FET in which an n-type source 2 and an n-type drain 3 are formed in a row by the diffusion of phosphorus into the surface of a p-type silicon substrate 1. The surface of the silicon substrate 1 is covered by a silicon dioxide film 5 having through-holes for the source 2 and the drain 3. Double layers of the silicon dioxide film (SiO.sub.2)5 and a silicon nitride film (Si.sub.3 N.sub.4)7d disposed on the silicon substrate 1 form between the source 2 and the drain 3, a gate insulating film 100. The silicon nitride film 7 serving to protect the FET device 11 covers a portion of the upper face of each of the conductive electrode films 6, which are formed on the silicon substrate 1 and the silicon dioxide film 5, and which come into contact with the source 2 and their drain 3 at the ends, respectively, which extend through the holes in the film 5. On the gate insulating film 100, the moisture sensitive means 9 and a moisture permeable gate electrode film 10 are successively formed. A blocking film 8 made of a conductive film is located between the moisture sensitive means 9 and the silicon nitride film 7. The blocking film 8 serves as an auxiliary electrode which applies a drift-cancellation voltage to the moisture sensitive means 9.
FIG. 6 shows an equivalent network of the above-mentioned FET type moisture sensor, wherein references Cs and Ci are the electrostatic capacities of the moisture sensitive means 9 and the double layered gate insulating film 100, respectively; reference R.sub.L is a load resistor connected in series with the drain electrode 6; and reference R.sub.B is a resistor connected in series with the blocking film 8.
The basic operation of the FET type moisture sensor is explained as follows: In order to simplify the explanation, the case that the moisture sensitive means 9 is directly formed on the gate insulating film 100 without the blocking film 8, that is, the resistor R.sub.B is omitted in the equivalent network in FIG. 6, is described, first.
Given that the voltage to be applied to the moisture permeable gate electrode film 10 is V.sub.A and the threshold voltage of the FET device 11 is V.sub.th, the drain current I.sub.D can be represented by the following equation (1): ##EQU1## wherein .mu.n is a carrier mobility; L and W are the channel length and the channel width of the FET device, respectively; and C is an electrostatic capacity, in the case where an electrostatic capacity Ci of the gate insulating film is connected in series with an electrostatic capacity C.sub.s of the moisture sensitive means 9, and is represented by the equation (2): ##EQU2##
Thus, given that the gate voltage V.sub.A is a constant value, moisture can be detected as a variation of the drain current I.sub.D with the variation of the electrostatic capacity C.sub.s of the moisture of the external atmosphere.
The role of the blocking film (i.e., auxiliary electrode film) 8 and the resistor R.sub.B in the operation of the moisture sensor is described below:
The blocking film 8 is connected with the moisture permeable gate electrode film 10 on the moisture sensitive means 9 by the resistor R.sub.B as shown in FIG. 6. A voltage V.sub.A, which is composed of a DC voltage V.sub.A (DC) and an AC voltage of frequency f superposed thereon, is applied to the gate insulating film 100 and the moisture sensitive means 9 through the moisture permeable gate electrode film 10 and the blocking film 8 to thereby drive this FET type moisture sensor. In the case where the DC voltage V.sub.A (DC) is smaller than the withstand voltage of the gate insulating film 100 and a leakage current does not occur through the gate insulating film 100, the DC voltage component V.sub.G (DC) of the effective gate voltage V.sub.G applied to the blocking film 8 becomes equal to the DC voltage V.sub.A (DC), resulting in no DC potential difference between both surfaces of the moisture sensitive means 9. The DC voltage V.sub.A (DC) functions to give an optimum bias voltage in the I.sub.D - V.sub.G characteristic of the FET device. In order that the FET type moisture sensor operates as a moisture sensor, that is, it detects the variation of the electrostatic capacity C.sub.s of the moisture sensitive means due to moisture in an atmosphere, the AC voltage V.sub.A (AC) is essential.
The AC voltage is described as follows:
In the case where the resistor R.sub.B, having a resistance value which is sufficiently great as compared with the impedance (2.pi.fC.sub.S).sup.-1 of the moisture sensitive means 9 at frequency f, is connected to the blocking film 8 and the moisture permeable gate electrode film 10, resistance of R.sub.B is negligible and the AC voltage component V.sub.G (AC) of the gate voltage V.sub.G can be represented by the equation (3): ##EQU3##
This indicates that since V.sub.G (AC) varies with the values of the electrostatic capacity C.sub.s of the moisture sensitive means at the application of V.sub.A (AC) with a given amplitude, the output signal required for a moisture sensor can be detected as the AC amplitude of the drain current I.sub.D. Thus, the modulation of an electric current flowing from the source 2 to the drain 3 while applying V.sub.A (AC) to the gate electrode film 10, can be detected by the conductive electrode film 6 as a detecting signal.
The output signal from the drain 3 can be represented by the equation (4): ##EQU4##
This indicates that the output signal depends upon the mutual conductance Gm of the FET device. It is well known that the Gm decreases with a rise in temperature and is different among FET devices.
Therefore, the FET type moisture sensor developed previously by the applicant has the problems that a wide variation of the characteristics among FET devices must be corrected by the regulation of circuits, and a wide variation of the output depending upon temperature must be corrected.